The present invention relates to determination of overlay between structures formed in single or multiple layers. More particularly, it relates to determining overlay based on radiation scattering from such structures.
In various manufacturing and production environments, there is a need to control alignment between various layers of samples, or within particular layers of such samples. For example, in the semiconductor manufacturing industry, electronic devices may be produced by fabricating a series of layers on a substrate, some or all of the layers including various structures. The relative position of such structures both within particular layers and with respect to structures in other layers is relevant or even critical to the performance of completed electronic devices.
The relative position of structures within such a sample is sometimes called overlay. Various technology and processes for measuring overlay have been developed and employed with varying degrees of success. More recently, various efforts have been focused on utilizing radiation scatterometry as a basis for overlay metrology.
Some approaches to determining overlay from scatterometry measurements concentrate on comparison of the measured spectra to calculated theoretical spectra based on model shape profiles, overlay, and film stack, and material optical properties (n,k dispersion curves), or comparison to a reference signal from a calibration wafer.
In another scatterometry approach for determining overlay, overlay targets having predefined offsets between the periodic structures of two different process layers are utilized. The offsets are generally selected so that overlay may be determined from the spectra measured from such target without using calculated theoretical or calibration data.
Although these scatterometry techniques work well under certain conditions, it is a continuing goal to strive towards improved scatterometry techniques, for example, that increase accuracy, efficiency, and/or minimize the area utilized for the overlay targets.